The pilot reported he was flying above the reservoir at 500 feet at approximately thirty minutes before sundown. As he completed a 180 degrees turn, the sun was in his eyes. He put on a cap to block the sun from his eyes, and as he was adjusting the cap, the aircraft impacted the water. The pilot stated he “apparently relaxed the back pressure on the stick enough to lose altitude.” During the impact, the right wing incurred substantial damage. No pre-impact malfunctions were reported by the pilot.